Ridge Complex Research Articles

A 3D regional crustal model of the NE Atlantic based on seismic and gravity data

Abstract We present a 3D regional crustal model for the North Atlantic, which is based on the integration of seismic constraints and gravity data. The model addresses the crustal thickness geometry, and includes information on sedimentary thickness, the presence of high-velocity zones in the lower crust, and information on the crustal density distribution in the continental and oceanic domains. Using an iterative forward- and inverse-modelling approach, we adhere to the seismic constraints within their uncertainty, but manage to enhance the crustal geometry in areas where seismic data are sparse or absent. A number of basins are resolved with more detail. Recently released seismic reflection data beneath the NE Greenland Shelf allowed for a major improvement of the crustal thickness estimates. Estimated Moho depths beneath the basins there vary between 15 and 25 km, which is compatible with the conjugate Norwegian margin. A major lower-crustal seismic velocity anomaly in the vicinity of the Greenland–Iceland–Faroe Ridge complex is supported by density modelling. We discuss the validity and uncertainties of our model assumptions and discuss the correlation with the main structural elements of the North Atlantic.

Geophysical analysis of the Alpha–Mendeleev ridge complex: Characterization of the High Arctic Large Igneous Province

The Alpha–Mendeleev ridge complex is a first-order physiographic and geological feature of the Arctic Amerasia Basin. High amplitude “chaotic” magnetic anomalies (the High Arctic Magnetic High Domain or HAMH) are associated with the complex and extend beyond the bathymetric high beneath the sediment cover of the adjacent Canada and Makarov–Podvodnikov basins. Residual marine Bouguer gravity anomalies over the ridge complex have low amplitudes implying that the structure has minimal lateral density variability. A closed pseudogravity (magnetic potential) contour around the ridge complex quantifies the aerial extent of the HAMH at ~1.3×106km2.We present 2D gravity/magnetic models for transects across the Alpha Ridge portion of the complex constrained with recently acquired seismic reflection and refraction data. The crustal structure is modeled with a simple three-layer geometry. Large induced and remanent magnetization components were required to fit the observed magnetic anomalies. Density values for the models were based on available seismic refraction P-wave velocities. The 3000kg/m3 lower crustal layer is interpreted as a composite of the original crustal protolith and deep (ultramafic) plutonic intrusions related to a plume sourced (High Arctic) LIP. The 2900kg/m3 mid-crustal and 2600kg/m3 upper-crustal layers are interpreted as the combined effect of sills, dikes, and flows. Volumetric estimates of the volcanic composition include (at least) 6×106km3 for the mid- and upper-crust and between 13×106 and 17×106km3 within the lower crust — for a total of ~20×106km3.We compare the magnetic structure, pseudogravity, and volumetric estimates for the HAMH portion of the HALIP with global large igneous province analogs and discuss implications for Arctic tectonics. Our results show that the closest analog to the HAMH/HALIP is the Kerguelen Plateau, which is considered a continental plateau intensively modified by plume-related volcanism.

Late Quaternary development of the Croatan Beach Ridge Complex, Bogue Sound, Bogue Banks, NC, USA and implications for coastal evolution

Foraminiferal, sedimentological, geophysical, and geochronologic data were utilized to elucidate the late Quaternary geologic development of the Croatan Beach Ridge Complex (CBRC), Bogue Sound, and Bogue Banks, North Carolina, USA. The CBRC is a relict beach ridge feature located on the mainland. It is separated from the modern barrier island, Bogue Banks, by Bogue Sound. Seventeen cores along shore-normal and shore-parallel transects provided material for sedimentologic and foraminiferal analysis and resulted in the recognition of seven depositional facies representing a variety of coastal depositional environments.Chronologic and depositional facies data suggest the CBRC was initiated during MIS 5a and rapid southward progradation produced a cape structure. Eolian reactivation of the upper sand of the CBRC occurred during the last glacial maximum (∼18 ka). The age of flood tide delta deposits in Bogue Sound suggests that the Holocene barrier island, Bogue Banks, had formed by ∼6 ka. Shoreface ravinement resulted in a shoreface landward of the present shoreline by ∼3.5 ka. Seaward and westward spit progradation of Bogue Banks began ∼1.7 ka and continued to ∼1.3 ka. Normal marine salinity conditions were present in Bogue Sound ∼1.1 ka, suggesting removal of at least the narrowest parts of the barrier island, coeval with a previously documented segmentation of the southern Outer Banks barrier islands. Previous work has linked this segmentation to climate warming and increased tropical storm activity during the Medieval Climate Anomaly.This study illustrates the complex response of this coastal system to Pleistocene and Holocene sea-level and climate change over two major sea-level cycles. In particular, the regional geomorphology during MIS5a and the Holocene sea-level highstand differ significantly and this, in large part, was controlled by the antecedent geologic framework, resulted in the contrasting more localized coastal geomorphic response.

Molecular dynamics simulation and bioinformatics study on chloroplast stromal ridge complex from rice (Oryza sativa L.).

BackgroundRice (Oryza sativa L.) is one of the most important cereal crops in the world and its yield is closely related to the photosynthesis efficiency. The chloroplast stromal ridge complex consisting of PsaC-PsaD-PsaE plays an important role in plant photosynthesis, which has been a subject of many studies. Till now, the recognition mechanism between PsaC and PsaD in rice is still not fully understood.ResultsHere, we present the interaction features of OsPsaC and OsPsaD by molecular dynamics simulations and bioinformatics. Firstly, we identified interacting residues in the OsPsaC-OsPsaD complex during simulations. Significantly, important hydrogen bonds were observed in residue pairs R19-E103, D47-K62, R53-E63, Y81-R20, Y81-R61 and L26-V105. Free energy calculations suggested two salt bridges R19-E103 and D47-K62 were essential to maintain the OsPsaC-OsPsaD interaction. Supportively, electrostatic potentials surfaces of OsPsaD exhibited electrostatic attraction helped to stabilize the residue pairs R19-E103 and D47-K62. In particular, the importance of R19 was further verified by two 500 ns CG-MD simulations. Secondly, this study compared the stromal ridge complex in rice with that in other organisms. Notably, alignments of amino acids showed these two salt bridges R19-E103 and D47-K62 also existed in other organisms. Electrostatic potentials surfaces and X-ray structural analysis strongly suggested the stromal ridge complex in other organisms adopted a similar and general recognition mechanism.ConclusionsThese results together provided structure basis and dynamics behavior to understand recognition and assembly of the stromal ridge complex in rice.Electronic supplementary materialThe online version of this article (doi:10.1186/s12859-016-0877-0) contains supplementary material, which is available to authorized users.

Evolution of neodymium isotopic signature of seawater during the Late Cretaceous: Implications for intermediate and deep circulation

Neodymium isotopic compositions (εNd) have been largely used for the last fifty years as a tracer of past ocean circulation, and more intensively during the last decade to investigate ocean circulation during the Cretaceous period. Despite a growing set of data, circulation patterns still remain unclear during this period. In particular, the identification of the deep-water masses and their spatial extension within the different oceanic basins are poorly constrained. In this study we present new deep-water εNd data inferred from the Nd isotope composition of fish remains and Fe–Mn oxyhydroxide coatings on foraminifera tests, along with new εNd data of residual (partly detrital) fraction recovered from DSDP Sites 152 (Nicaraguan Rise), 258 (Naturaliste Plateau), 323 (Bellinghausen Abyssal Plain), and ODP Sites 690 (Maud Rise) and 700 (East Georgia Basin, South Atlantic). The presence of abundant authigenic minerals in the sediments at Sites 152 and 690 detected by XRD analyses may explain both middle rare earth element enrichments in the spectra of the residual fraction and the evolution of residual fraction εNd that mirror that of the bottom waters at the two sites. The results point towards a close correspondence between the bottom water εNd values of Sites 258 and 700 from the late Turonian to the Santonian. Since the deep-water Nd isotope values at these two sites are also similar to those at other proto-Indian sites, we propose the existence of a common intermediate to deep-water mass as early as the mid-Cretaceous. The water mass would have extended from the central part of the South Atlantic to the eastern part of proto-Indian ocean sites, beyond the Kerguelen Plateau. Furthermore, data from south and north of the Rio Grande Rise–Walvis Ridge complex (Sites 700 and 530) are indistinguishable from the Turonian to Campanian, suggesting a common water mass since the Turonian at least. This view is supported by a reconstruction of the Rio Grande Rise–Walvis Ridge complex during the Turonian, highlighting the likely existence of a deep breach between the Rio Grande Rise and the proto-Walvis Ridge at that time. Thus deep-water circulation may have been possible between the different austral basins as early as the Turonian, despite the presence of potential oceanic barriers. Comparison of new seawater and residue εNd data on Nicaraguan Rise suggests a westward circulation of intermediate waters through the Caribbean Seaway during the Maastrichtian and Palaeocene from the North Atlantic to the Pacific. This westward circulation reduced the Pacific water influence in the Atlantic, and was likely responsible for more uniform, less radiogenic εNd values in the North Atlantic after 80Ma. Additionally, our data document an increasing trend observed in several oceanic basins during the Maastrichtian and the Palaeocene, which is more pronounced in the North Pacific. Although the origin of this increase still remains unclear, it might be explained by an increase in the contribution of radiogenic material to upper ocean waters in the northern Pacific. By sinking to depth, these waters may have redistributed to some extent more radiogenic signatures to other ocean basins through deep-water exchanges.

Coastal Evolution Over the Past 3000 Years at Conrads Beach, Nova Scotia: the Influence of Local Sediment Supply on a Paraglacial Transgressive System

Many coastlines are retreating in response to sea level rise, compounded by glacial–isostatic subsidence in areas marginal to former ice sheets. The resulting barrier and estuarine deposits are dominated by transgressive stratigraphy. Where supplied primarily from relict glacial deposits, this “paraglacial” sediment input may rise and fall, increasing as a new source such as a drumlin headland is exposed to erosion but declining as the source becomes exhausted. Conrads Beach, on the Atlantic coast of Canada, has experienced a succession of barrier growth and reworking as sediment supply from several drumlin sources has varied over the past 3000 years. In the context of long-term regional transgression, there have been intervals of years to centuries characterized by local stability or progradation. Ground-penetrating radar profiles and refraction seismic data were used to image the facies architecture of Conrads Beach to depths of 6–8 and 10–24 m, respectively. Thirteen vibracores provided a record of lithofacies characteristics and geometry. Results show evidence of an estuarine basin at ~2800 years BP. As the outer coast retreated, erosion of drumlins provided multi-century sediment pulses to adjacent beaches and embayments. Locally increased sediment supply fed a prograding beach ridge complex from >600 to ~150 years BP and tidal channels feeding sediment to back-barrier flood delta deposits. This study documents the complexity of coastal adjustment to time- and source-varying sediment supply under long-term rising sea level. It expands and refines previous models, providing guidance required for effective management and hazard mitigation on transgressive paraglacial coasts.

A tale of two basins: An integrated physical and biological perspective of the deep Arctic Ocean

A tale of two basins: An integrated physical and biological perspective of the deep Arctic Ocean

Re-examination of geophysical data off Northwest India: Implications to the Late Cretaceous plate tectonics between India and Africa

Abstract The Gop and Laxmi Basins lying off Northwest India have been assigned ambiguous crustal types and evolution mechanisms. The Chagos–Laccadive Ridge (CLR) complex lying along the southwest coast of India has been attributed to different evolutionary processes. Late Cretaceous seafloor spreading between India and Africa formed the Mascarene Basin, and the plate reconstruction models depict unequal crustal accretion in this basin. Re-interpretation of magnetic data in the Gop and Laxmi Basins suggests that the underlying oceanic crust was accreted contemporaneously from 79 Ma at slow half spreading rates (0.6 to 1.5 cm/yr) separating the Seychelles–Laxmi Ridge complex from India. The spreading ridge became extinct at 71 Ma in the central region between 19–20°N and 65.5–67°E, and at 68.7 Ma in the Gop Basin. Extinction progressed southwards with time until 64.1 Ma at ~ 14.5°N in the Laxmi Basin. This spreading probably limited the seafloor spreading in the northern Mascarene Basin, while normal to fast spreading continued in the south. The fracture zone FZ2 acted as a major boundary between the northern and southern Mascarene Basin, and may have also restricted the southward propagation of the Laxmi Basin spreading ridge. Plate reconstruction models generated in this study accommodate several microplates between Madagascar and India, and trace their movements over time within the India–Africa spreading corridor. Furthermore, these models suggest the formation of the Seychelles–Laxmi Ridge microplate by 79 Ma in a complex three plate system. This microplate may have existed up to 64.1 Ma and experienced an anticlockwise rotation of about 50°. Under the influence of the Deccan hotspot activity, the Laxmi Basin spreading ridge jumped southwestward splitting the Seychelles–Laxmi Ridge complex and initiating the Carlsberg Ridge. The presence of about 160 km of oceanic crust in the Gop and Laxmi Basins partially reduces the space problem faced in the earlier plate reconstruction models.

Vulcanolepas scotiaensis sp. nov., a new deep-sea scalpelliform barnacle (Eolepadidae: Neolepadinae) from hydrothermal vents in the Scotia Sea, Antarctica.

A new deep-sea stalked barnacle, Vulcanolepas scotiaensis sp. nov. is described from hydrothermal vents at depths of 2400-2600 metres along segments of the East Scotia Ridge and from 1400 metres in the Kemp Caldera. Both locations are areas of volcanic activity that lie on the Antarctic-South American Ocean Ridge complex near the South Sandwich Islands. This discovery confirms a wide distribution in southern seas for Vulcanolepas, complementing the previous records from deep-sea vents in the Lau Basin and Kermadec Ridge in the southwest Pacific, and the Pacific Antarctic Ridge in the southeast Pacific. V. scotiaensis sp. nov., the third described species of Vulcanolepas shows an extraordinary range in morphology, requiring a reassessment of the original diagnosis for Vulcanolepas. Although the morphological envelope of V. scotiaensis sp. nov. includes representatives with a peduncle to capitulum ratio similar to that observed in most neolepadines, the peduncle generally shows greater proportional length than in species in any neolepadine genus except Leucolepas; it is distinguished from other species of Vulcanolepas by a broader capitulum, much smaller imbricating scales on the peduncle and more ornamented capitulum plates. The morphological diversity of V. scotiaensis sp. nov. is interpreted as having arisen due to abrupt changes in water temperature.LSID: urn:lsid:zoobank.org:act:AA2AFDA5-0B08-466A-A584-D3FDBDE9DA61.

40Ar/39Ar Geochronology of Subaerial Ascension Island and a Re-evaluation of the Temporal Progression of Basaltic to Rhyolitic Volcanism

Ar/Ar geochronology of basaltic to rhyolitic lavas, domes, and pyroclastic deposits from Ascension Island indicates that the maximum age of subaerially exposed samples is 1094 ka. Thirty-eight Ar/Ar ages, coupled with new and existing geochemical data, constrain the eruptive histories of the four distinct mafic magma types (high Zr/Nb, low Zr/Nb, intermediate Zr/Nb, and Dark Slope Crater) and document temporal variations in magma sources. Lavas from the eastern felsic complex, previously assumed to be as old as or slightly younger than the 602^1094 ka Middleton Ridge complex, are as young as 52 ka. Basaltic to benmoreitic scoria cones and associated flows of the intermediate Zr/Nb magma type have been inferred to be the most recent eruptive products, yet their eruptive histories extend back to 705 ka. These intermediate Zr/ Nb magmas are likely to be parental to the abundant trachytic to rhyolitic lavas and domes, which contradicts previous interpretations that call upon a high Zr/Nb parental basalt. Two distinct fractionation trends are observed in the trace element variations of Ascension trachytes and rhyolites. Ar/Ar ages of the samples defining the two trends suggest that ilmenite fractionation dominated the Nb budget and thus controlled Zr/Nb ratios in early (4931ka) Ascension evolved magmas, whereas zircon^titanite fractionation was predominant in younger felsic magmas.The eruptive sequence and compositions of the subaerial lavas and domes at Ascension Island are unique in comparison with other ocean island volcanoes because of its on-axis location and eruptions of highSiO2 trachyte and rhyolite.

Interpretation of Coastal Morphodynamics of Subarnarekha Estuary Using Integrated Cartographic and Field Techniques

Cartographic techniques are frequently applied for coastal mapping, but their application for the study of coastal morphodynamics is unpopular, as they only give limited parameters like plan-view, geometry, area-length measurement and lithology. On the other hand, detailed study of morphodynamics requires extra information like depthwise variation in lithology and absolute dating. Since both these techniques are expensive, cartographic techniques can be cost-effective supplementary. In the present study, morphodynamic history of Subarnarekha estuary from 7000 years BP has been interpreted applying data from cartographic techniques along with shallow wells, 14 C-dating and literature survey. Geomorphologic features of the coastal plain were identified in the order of hierarchy, viz. chenier plain (first order); beach ridge complex, spit complex, chenier complexes (second order); cheniers (third order); and simplest ridge, spit, washover beach (fourth order). Following this field-investigated geological history of river dynamics (both Ganges and Subarnarekha) and sea-level changes identified by earlier researchers were merged with cartographically observed features. The studied ridge chronology provides six sequences of chenier complex development agewise, whereas geometry of spit complexes suggests chronological conversion of Subarnarekha estuary from initial wave-dominated to tidedominated flow.

The Soapstone Ridge Complex, Southern Appalachians: petrographic, mineral compositional, and oxygen isotope investigation

Samples of the Soapstone Ridge Complex (SSR) in the Southern Appalachians examined for this study are composed of megacrystic orthopyroxenites, amphibole schists, and talc-chlorite-amphibole schists. The mineralogy of the SSR samples studied consists of varying proportions of amphibole, chlorite, talc, orthopyroxene, and Fe-Ti oxides. Amphibole is the domi- nant mineral in almost all SSR samples studied. The amphiboles display zoning, whereby cores that are composed of magne- siohornblende and edenite, consistent with amphibolite facies conditions, are rimmed by tremolite/actinolite, consistent with greenschist facies conditions. Oxygen isotope compositions of SSR amphiboles investigated show a wide range in 18O values, from a low value of 1.8‰ to a high value of 6.1‰. Such wide ranges in 18Oamphibole values occur in ophiolite complexes and oceanic crustal rocks. Relict orthopyroxene from the SSR has a 18O value of 3.4‰, a value lower than those from layered intrusions but comparable to some pyroxene 18O values from ophiolites. Equilibration temperatures calculated for actinolite- chlorite yield a temperature of 227 °C, consistent with greenschist facies conditions. It is proposed that the SSR amphibole cores were metamorphosed probably during terrane-terrane collision under amphibolite facies conditions, with the actinolite rims possibly being formed during thrusting of the SSR rocks onto the continental margin.

Geomorphological implications of the basement structure in the Krishna-Godavari deltas, India

Summary. A correlation of the morphology of the Krishna-Godavari twin delta region with the tectonic framework of its basement along the east coast of India revealed that the Kolleru Lake in the inter-delta region is situated over a basement graben known as Gudivada Sub-basin, while the vast mudflat to the south of the lake coincides with the Bantumilli Sub-basin. The 8-km-wide beach ridge complex that separates the Kolleru and the southern mudflat largely coincides with the Kaja-Kaikaluru Ridge (horst) that lies in between the Gudivada and Bantumilli Sub-basins (grabens). The anomalous landward meandering of the Vasishta and Vainateyam distributaries close to the coastline and the three-fold widening during the past four decades of the Nilarevu estuary in the Godavari delta are in line with the Matyapuri-Palakollu Fault. Similarly, the 70-km-long straight segment of the delta-front coast between the Gautami and Vasishta distributary mouths in the Godavari delta, which is rather anomalous for a prograding delta, coincides with the landward boundary of the Ravva offshore sub-basin. Swerving of the Krishna River course throughout its deltaic reaches and the clock-wise bending of the active Krishna delta lobe into the Nizampatnam offshore sub-basin appear to be associated with the Krishna Cross Trend fault. Furthermore, the elongated nature of the Krishna delta and its visibly greater seaward bulge than its bigger neighbour (Godavari delta) in spite of the fact that the latter receives higher sediment loads and water discharges, might also be due to tectonic influence. Although the connection between the surface morphology and basement framework is hard to establish for want of detailed shallow subsurface data, the landform anomalies point to the possible influence of neotectonics in shaping the morphologies of the Krishna-Godavari deltas.

On the origin of the Amerasia Basin and the High Arctic Large Igneous Province—Results of new aeromagnetic data

The history of the 2.5 million km2 Amerasia Basin (sensu lato) is in many ways the least known in the global tectonic system. Radically different hypotheses proposed to explain its origin are supported only by inconclusive and/or indirect observations and several outstanding issues on the origin of the Basin remain unaddressed. The difficulty lies in the geodynamic evolution and signature of the Basin being overprinted by excess volcanism of the Alpha–Mendeleev Ridge complex, part of the High Arctic Large Igneous Province (HALIP) and one of the largest (>1 million km2) and most intense magmatic and magnetic complexes on Earth. Here, we present the results of a 550,000km2 aerogeophysical survey over the poorly explored Lomonosov Ridge (near Greenland) and adjoining Amerasia and Eurasia Basins that provides the first direct evidence for consistent linear magnetic features between the Alpha and Lomonosov Ridges, enabling the tectonic origin of both the Amerasia Basin and the HALIP to be constrained. A landward Lower Cretaceous (∼138–125(120)Ma) giant dyke swarm (minimum 350×800km2) and tentative oceanward Barremian (or alternatively lower Valanginian–Barremian) seafloor spreading anomalies are revealed. Prior to Cenozoic opening of the Eurasia Basin the giant dyke swarm stretched from Franz Josef Land to the southern Alpha Ridge and possibly further to Queen Elisabeth Islands, Canada. The swarm points towards a 250-km-wide donut-shaped anomaly on the southern Alpha Ridge, which we propose was the centre of the HALIP mantle plume, suggesting that pronounced intrusive activity, associated with an Alpha Ridge mantle plume, took place well before the Late Cretaceous Superchron and caused continental breakup in the northern Amerasia Basin. Our results imply that at least the southern Alpha Ridge as well as large parts of the area between the Lomonosov and southern Alpha Ridges are highly attenuated continental crust formed by poly-phase breakup with LIP volcanic addition. Significantly, our results are consistent with an early (∼pre-120Ma) overall continental scale rotational opening of the Amerasia Basin in which the Eurasian continental margin is rifted from the Canadian–southern Alpha Ridge margin about one or more poles in the Mackenzie Delta (Alaska). The findings provide a key to resolving Arctic plate reconstructions and LIPs in the Mesozoic.

Erosion, deposition and landscape change on the Sendai coastal plain, Japan, resulting from the March 11, 2011 Tohoku-oki tsunami

Case studies of recent tsunami impacts have proven to be extremely useful in understanding the geologic processes involved during inundation and return flow, and refining the criteria used to identify paleotsunami deposits in the geologic record. Here, we report on erosion, deposition and associated landscape change resulting from the March 11, 2011 Tohoku-oki tsunami along a nearly 4.5km shore-normal transect on the coastal plain near Sendai, Japan. The study area on the broad, low-relief prograding coastal Sendai plain comprised a sand beach backed by ~3m high sand dunes and a forest, a wetland, the Teizan canal, agricultural rice fields, buildings and roads.Field observations focused on measurements of tsunami flow characteristics (height and direction), mapping of erosion features and assessing sediment deposition based on shallow trenches at 50–100m spacing. Recorded tsunami inundation heights reached up to about 11m above mean sea level within the first 500m from the shoreline and then ranged between 3 and 5m for the next 2km, gradually decreasing to about 3m close to the inundation limit. The tsunami deposit generally thinned landward from an average maximum ~30cm thick sand deposit in the coastal forest to a thin mud drape several mm thick near the inundation limit. A discontinuous sand-dominated sheet was prevalent to about 2800m from the shoreline where mud content then gradually increased further landward eventually resulting in a mud-dominated deposit ranging from 3.5cm to a few mm thickness. The overall thinning and fining of the deposit was often interrupted by localized features that led to complex sedimentological relationships over short distances.Satellite imagery taken on 14 March 2011, 3days after the Tohoku-oki Tsunami shows prominent foreshore incisions with 100s+meters spacing alongshore, a foredune ridge that underwent severe erosion and development of a prominent shore-parallel elongated scour depression. Our field survey in early May 2011 revealed that the foreshore recovered quickly with rapid post-tsunami sediment deposition from incident waves, whereas the dune–ridge complex had undergone only minor re-working from eolian processes.

Evolution of Macquarie Ridge Complex seamounts: Implications for volcanic and tectonic processes at the Australia–Pacific plate boundary south of New Zealand

The Macquarie Ridge Complex (MRC) coincides with the Australia–Pacific plate boundary south of New Zealand and is characterised by a rugged bathymetry that comprises numerous seamounts along its length. Tectonic plate reconstructions show that the Australia–Pacific plate boundary evolved from a spreading centre to a transpressional boundary from ca. 40 to 6Ma. However, limited sampling and high-resolution mapping of the region have meant that the composition and morphology of the MRC remain relatively unknown. Here we present results from multibeam mapping, photographic investigation and petrologic and geochemical analyses from 10 seamounts along the MRC. Dredge samples collected from the seamounts are alkaline to sub-alkaline basalts, containing 45.1–50.8wt.% SiO2, 3.5–12.0wt.% MgO and 1.9–4.9wt.% total alkalis. All samples are enriched mid-ocean ridge basalts that have K/Ti>0.15, which originated as low degree partial melts during the late stages of mid-ocean ridge volcanism. This oceanic crust has been sheared and accreted along the plate boundary since ca. 6Ma by cumulative transpressional relative plate motion, such that lavas from distinct magma chambers have been juxtaposed at individual seamounts along the ridge. MRC seamounts have formed as discrete elevations as a result of dip-slip and strike-slip faulting of the ridge axis. Three guyot-type seamounts have flat-top summit plateaux that were presumably formed by wave erosion via relative sea-level lowering due to uplift of the ridge. These seamounts have since subsided due to extension associated with the step-over fault geometry of the transform plate boundary along the McDougall and Macquarie ridge segments. While the seamounts are of volcanic origin, they are manifestations of tectonic and geomorphic processes. Seamounts distal from the ridge axis, however, are not subjected to plate boundary deformation and instead have morphologies formed through volcanic processes.

The nature of the acoustic basement on Mendeleev and northwestern Alpha ridges, Arctic Ocean

The Alpha-Mendeleev ridge complex, over 1500 km long and 250–400 km wide, is the largest submarine structure in the Arctic Ocean basin. Its origin is unknown, but often inferred to represent a large igneous province where domains of continental crust may also be a possibility. We investigate the basement geology of part of this large scale feature using 1100 km of multichannel seismic reflection data, sonobuoy recordings and marine gravity data acquired in 2005 from USCG icebreaker Healy. The sonobuoy results show top and intra-acoustic basement velocities in the range of 2.3–4.0 km/s and the seismic reflection attributes define three main acoustic facies: 1) continuous high amplitude reflections often with abrupt breaks, 3) weak wedge geometry and 3) segmented, disrupted to chaotic reflections. The acoustic characteristics and seismic velocities compare more closely with basement on Ontong Java Plateau than normal ocean crust or wedges of seaward dipping reflections at volcanic margins. The acoustic facies are interpreted to represent basalt flows and sills capping voluminous tuff deposits and possible sediments. At least two volcanic centres are identified. The upper volcanic carapace on the surveyed part of Mendeleev and northwestern Alpha ridges was emplaced during a brief igneous episode no later than Campanian (80 Ma) and most likely part of wider Late Cretaceous circum Arctic volcanism. The horst and graben morphology on Mendeleev Ridge is largely a result of post-emplacement faulting where a number of the major extensional faults remained active until a late Miocene intrusive event.

Phylogenetic Characterization of Episymbiotic Bacteria Hosted by a Hydrothermal Vent Limpet (Lepetodrilidae, Vetigastropoda)

Marine invertebrates hosting chemosynthetic bacterial symbionts are known from multiple phyla and represent remarkable diversity in form and function. The deep-sea hydrothermal vent limpet Lepetodrilus fucensis from the Juan de Fuca Ridge complex hosts a gill symbiosis of particular interest because it displays a morphology unique among molluscs: filamentous bacteria are found partially embedded in the host's gill epithelium and extend into the fluids circulating across the lamellae. Our objective was to investigate the phylogenetic affiliation of the limpet's primary gill symbionts for comparison with previously characterized bacteria. Comparative 16S rRNA sequence analysis identified one γ- and three ε-Proteobacteria as candidate symbionts. We used fluorescence in situ hybridization (FISH) to test which of these four candidates occur with the limpet's symbiotic gill bacteria. The γ-proteobacterial probes consistently hybridized to the entire area where symbiotic bacteria were found, but fluorescence signal from the ε-proteobacterial probes was generally absent. Amplification of the γ-proteobacterial 16S rRNA gene using a specific forward primer yielded a sequence similar to that of limpets collected from different ridge sections. In total, direct amplification or FISH identified a single γ-proteobacterial lineage from the gills of 23 specimens from vents separated by a distance up to about 200 km and collected over the course of 2 years, suggesting a highly specific and widespread symbiosis. Thus, we report the first filamentous γ-proteobacterial gill symbiont hosted by a mollusc.

Seismic stratigraphy and deformational styles of the offshore Cyrenaica (Libya) and bordering Mediterranean Ridge

Off northwest Libya the Cyrenaica foreland basin domain and its Pan-African continental crust, which constitute the African promontory, are overthrusted by the Mediterranean Ridge Complex. The thrust belt contact and its seismic stratigraphy have been analysed using pre-stack depth-migrated multichannel seismic (MCS) lines recorded during the MEDISIS survey (2002). The geometry and sedimentary distribution analysis through the wedge-top depocentres allow reconstruction of schematic cross-sections of the tectono-sedimentary wedge that includes two major thrust sequences separated by an apparently poorly deformed transition zone. Based on time–space variation of several piggyback basins, we propose that these thrust sequences relate to distinct phases of shortening. (1) A first event, which probably occurred just prior to the Messinian crisis in latest Miocene (Tortonian times?) and (2) A second event, that has finally led to the present-day overthrusting of the Mediterranean Ridge over the Libyan continental slope.

Tanaidacea (Crustacea, Peracarida) of the North-east Atlantic: the Agathotanaidae of the AFEN, BIOFAR and BIOICE projects, with a description of a new species of Paragathotanais Lang

The tanaidacean fauna of the Iceland-Faroes-Shetlands sector of the North-east Atlantic was studied using material from the BIOFAR, BIOICE and AFEN surveys. Seven agathotanaid species were recorded from the genera Agathotanais, Paragathotanais and Paranarthrura, a total comparable to those from a similar bathymetric range in the Rockall-Biscay area and the Gulf of Mexico. A new species of Paragathotanais is described. All records of this family were from benthic stations with a mean bottom temperature of >= 2 degrees C, with none from the 'cold-water' region north of the Iceland-Shetlands ridge complex. An analysis of North-east Atlantic agathotanaid sex-ratios shows that these are not highly skewed in favour of females and are consistent with the status of the males as relatively non-dimorphic and long-lived entities.